When separating gaseous pollutants from process gases, such as flue gases from a coal- or oil-fired fired power plant, a method is frequently used, in which a lime-containing absorbent material is introduced into the process gas to react with the gaseous pollutants. When the absorbent material reacts with the gaseous pollutants, the gaseous pollutants are converted chemically or physically into dust, which is then separated in a filter.
WO 96/16722 discloses a method, in which lime-containing dust is mixed with water in a mixer and then introduced into a contact reactor to react with gaseous pollutants in a flue gas. Then the dust is separated in a filter and recirculated to the mixer to be mixed once more with water and subsequently again be introduced into the contact reactor. Thus, the dust will be circulated through the mixer several times. In each circulation, a small amount of dust is removed from the filter, and a small amount of fresh lime-containing material, for instance burnt lime, CaO, or dust containing a certain amount of burnt lime, is supplied to the mixer.
It has now been found that the above method results in a low degree of utilisation of the burnt lime supplied, i.e. that also the dust removed from the filter contains some burnt lime, in certain types of absorbent material. This increases the consumption of burnt lime and implies that the dust removed from the filter will contain a great deal of burnt lime that has not reacted. The low degree of utilisation of the burnt lime increases the costs of operating the plant and makes the handling of removed dust difficult.